Technical Field
The present invention generally relates to a process for in situ upgrading of a heavy hydrocarbon in the presence of one or more asphaltene precipitant additives.
Description of the Related Art
Subsurface upgrading of heavy oil (HO) has always been of interest to the petroleum industry mainly because of the intrinsic advantages compared with aboveground counterparts. The main advantages are: lower lifting and transportation costs from the underground to the refining centers with the potential increase of the volumetric production of wells and in the value of the upgraded oil, decrease in consumption of costly light and medium petroleum oils used as solvents for HO production, move estimated and probable HO reserves to proved reserves, possibility of reducing capital and operating expenses of Upgrader Units by performing the upgrading subsurface and use of porous media (a mineral formation) as a natural chemical “catalytic reactor” to further improve the properties of upgraded crude oil. However, there are significant challenges that have to be addressed to accomplish a successful downhole upgrading process.
A number of methods have been proposed for producing such heavy hydrocarbons. These methods include the use of multiple wells including parallel horizontal wells drilled into water formations beneath the heavy hydrocarbon, and injection of various additives through the horizontal wells so as to assist in production from a separate well drilled into the producing formation. Unfortunately, this method requires the actual drilling of a potentially large number of wells for production from a single well, with the attendant increase in cost of labor and equipment, and also results in large amounts of potentially expensive additives being injected through the horizontal wells.
“Huff and puff” or cyclic pressurizing and production techniques are also known wherein a well is pressurized for a period of time, and then allowed to produce. This method can provide enhanced production for certain wells. However, with particularly heavy hydrocarbons, this technique certainly leaves room for improvement.
In SPE paper No. 25452, a process for the in-situ upgrading of heavy oils and bitumen by propane deasphalting is proposed. This process utilizes two parallel horizontal wells, as reported in the Steam Assisted Gravity Drainage process (SAGD), but with the steam chamber being replaced by a chamber containing hydrocarbon vapor near its dew point. In this process, cold propane is continuously injected for the top horizontal well and the upgraded heavy oil/solvent blend is produced from the bottom well. The heavy oil is upgraded in terms of permanent viscosity reduction via solvent deasphalting.
Gupta and Gittins (Conference paper No. 2005-190 presented at Canadian International Petroleum Conference, Jun. 7-9, 2005, Calgary, Alberta) reported the field testing of a solvent aided process which involves the co-injection of a hydrocarbon solvent and steam during SAGD operation. The authors observed increases up to one degree of the API gravity of the produced oil.
In U.S. Pat. No. 6,883,607 (“the '607 patent”), a process for the recovery of hydrocarbons is disclosed which involves the use of warm solvents to extract heavy oil from oil bearing formation. In the '607 patent, the solvent is continuously injected downhole and placed into the formation at a temperature and pressure sufficient for the solvent to be in the vapor state and to condense on the extraction surface. Then, a solvent-heavy oil blend is produced and, after solvent separation and purification, it is re-injected into the formation again. The '607 patent further discloses that the presence of the solvent in the heavy oil leads to precipitation of asphaltenes which upgrades the heavy oil via improvements in the API and reduction of metals and sulfur contents and Conradson carbon.
Another example is U.S. Pat. No. 6,405,799 (“the '799 patent”) which discloses a process for in situ upgrading of a heavy hydrocarbon. The process disclosed in the '799 patent includes the steps of (a) positioning a well in a reservoir containing a heavy hydrocarbon having an initial API gravity of less than or equal to about 8; (b) injecting a light solvent into the well at reservoir conditions so as to provide an upgraded hydrocarbon in the reservoir, the upgraded hydrocarbon having an improved API gravity greater than the initial API gravity; and (c) producing the upgraded hydrocarbon from the well.
In all of the mentioned prior art, the use of large amounts of solvent is required in order to precipitate asphaltenes downhole to upgrade of the heavy oil, as measured by, for example, API gravity increase and permanent viscosity reduction. Independent of the type of process, for example, Huff and Puff (discontinuous) or continuous solvent injection such as SAGD, the solvent to produced-heavy oil ratios used in the field are in the range of 0.5 up to 10 (v/v). These high solvent-to-heavy-oil ratios not only increase the operating expenses of the process due to the need of high solvent inventories but also increase the capital costs due to larger size surface facilities for solvent separation, purification and recycling. Additionally, due to loss of injected solvent to thief or cracked zones present in the reservoir, there is a need for solvent make-up that further increases the operating costs of the downhole upgrading processes.
Accordingly, it would be desirable to provide improved processes for in situ upgrading of heavy crude oils that can be carried out with lower operating and capital expenses in a simple and cost efficient manner.